Method of electric discharge machining a cathode for an electron gun

ABSTRACT

A method of machining a cathode used in an electron gun. The method includes steps of placing an object to be machined, which is a material of the cathode, and a machining electrode in an electrically insulating oil; applying a discharge current to the machining electrode; and machining the object to be machined to have a shape corresponding to the shape of the machining electrode by means of electric discharge machining of the machining electrode. The machining electrode includes a disk, recesses formed on a front surface of the disk, a protrusion provided on a back surface of the disk, and a hole for mounting a lead wire for applying discharge current to the machining electrode. Bumps corresponding to the shape of the recesses of the machining electrode are formed on the object to be machined by means of the electric discharge machining.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional application of U.S. patent applicationser. No. 10/697,647, filed Oct. 31, 2003, now U.S. Pat. No. 7,205,559,and wherein application Ser. No. 10/697,647 is a Continuation ofInternational Application No. PCT/JP02/04327, filed Apr. 30, 2002, theentire disclosures of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a multi-beam type electron beamapparatus, and a semiconductor device manufacturing method which usessuch an apparatus to evaluate wafers in the middle of processes forimproving the yield. More particularly, the present invention relates toimprovements on the configuration of a secondary optical system, acathode, and an electron gun of a multi-beam type electron beamapparatus.

BACKGROUND ART

Generally, when aberration of an optical system must be limited to acertain value or lower, the optical system is provided with a diaphragmsuch that the aperture diameter of the diaphragm is adjusted to make theoptical system brighter or to improve the resolution of the opticalsystem. Also, when a plurality of beams are handled, a diaphragm isprovided at a position at which a principal ray of the plurality ofbeams intersect with each other, i.e., a cross-over position in aprimary optical system.

As described above, while a diaphragm is provided at a cross-overposition in a primary optical system for handling a plurality of beams,problems described below arise if the diaphragm is provided in asecondary optical system. Specifically, since the cross-over positioncannot be previously predicted in the secondary optical system, aproblem arises in that the diaphragm cannot be matched with thecross-over position unless an adjuster lens is provided for adjustingthe cross-over position to match the position of the diaphragm. Anotherproblem is experienced when the diaphragm does not match the cross-overposition, in which case some of a plurality of secondary electron beamscloser to the optical axis are blocked by the diaphragm, so that eventhough the diaphragm is used to limit aberration of the optical system,the aberration cannot be reduced, resulting in a lower secondary beamdetection efficiency and inability to eliminate cross-talk with adjacentbeams.

As such, for providing a diaphragm in the secondary optical system, itis necessary to additionally provide a lens for focusing a cross-overimage on the diaphragm, and an adjustor lens such as a two-stage lensfor adjusting the dimensions of the cross-over at the diaphragm.However, an adjustor lens, if provided, will result in a longer opticalsystem and require an aligner for the adjustor lens. A further problemarises as to the need for an aligner for the diaphragm, and anastigmatism adjuster for the cross-over, resulting in a complicatedconfiguration of the diaphragm, a larger size of the overall apparatus,a longer time needed for adjustments, and a higher cost.

Thus, the utilization of a plurality of electron beams, i.e., multiplebeams has been proposed for testing a mask pattern or a wafer fordefects of LSI patterns and the like thereon at a high throughput. Forexample, a technology has been proposed for irradiating a plurality ofregions on an object under testing with respective electron beams inorder to improve throughput in such a defect detection. Also, anotherproposition has been made to the use of a field emission cathode whichis capable of producing a large electron beam current at a low voltagewhen a fine pattern on the order of 0.1 micron is tested for defectsusing a low-energy electron beam.

However, when an array of field emission cathodes, which are inherentlyinstable in operation, is used for an electron gun in a defect detectionapparatus to generate multiple beams, even one field emission cathode inthe array incapable of emission would cause the apparatus itself to failto operate, possibly resulting in a significantly reduced operatingrate.

Also, the instable operation of the field emission cathodes as mentionedabove causes problems of difficulties in identifying fluctuations inemissions from the field emission cathodes and signals, andparticularly, difficulties in providing an image with a largesignal-to-noise ratio due to large shot noise.

In addition, a conventional electron gun of the multi-beam type electronbeam apparatus has the following problems. FIG. 1 is a verticalsectional view schematically illustrating an exemplary conventionalelectron gun 100. An insulating ceramics 108 is supported within acylindrical electron gun body 106. A bottom face of a ceramic seat 109is fixed on a top face of the insulating ceramics 108. A single emitter101 is fixed on a top surface of the ceramic seat 109 such that it isheated by a heater 3 which is a heating means. A lead for the heater103, and high voltage cables 107 for a cathode extend from a bottom faceof the insulating ceramics 108.

A Wehnelt member, i.e., Wehnelt electrode 102 is fitted over thecylindrical electron gun body 106. The Wehnelt electrode 102 has one end(upper end in the figure) integrally formed with an end wall which isprovided with a single small hole (Wehnelt hole) 110.

The Wehnelt electrode 102 is fixed by a stop ring 104 at a position atwhich its end wall section is in close proximity to the emitter 101. Theinsulating ceramics 108 can be finely adjusted in its position in thehorizontal direction by a plurality of finely movable screws 105 whichextend through a peripheral wall of the electron gun body 106. Throughthe adjustment, the emitter 101 supported by the ceramic seat 109 on theinsulating ceramics 108 is brought into alignment with the hole 110provided through the end wall of the Wehnelt electrode 102.

However, there are problems in applying the method of finely adjustingthe relative position between the single emitter 101 and Wehneltelectrode 102 as mentioned to an electron gun which comprises amulti-emitter having a plurality of emitters.

First, when the insulating ceramics is finely moved to finely adjust themulti-emitter in its in-plane position through the ceramic seat, themulti-emitter can be inclined. While the inclination does not constitutea grave problem for the single emitter, the inclined multi-emitter wouldresult in different distances between the Wehnelt electrode and allemitters, and accordingly inconsistent emissions of electrons.

A second problem, which is also true for the single emitter, is thatwhen the entire Wehnelt electrode is moved to adjust the axial distancebetween the Wehnelt electrode and multi-emitter, the Wehnelt electrodecan be bumped against the multi-emitter which could thereby be broken.

DISCLOSURE OF THE INVENTION

The present invention has been proposed to solve the conventionalproblems mentioned above, and the present invention essentially providesan electron beam apparatus which is capable of reliably evaluating asample such as a wafer, a mask and the like having a pattern with theminimum line width of 0.1 μm or less at a high throughput using aplurality of electron beams, i.e., multiple beams, an electron beamapparatus which comprises an electron gun for generating multiple beamsusing a plurality of emitters, particularly, an electron gun which iscapable of accurately and readily making fine adjustments of relativepositions between the plurality of emitters and a Wehnelt electrodewhich has holes corresponding to the plurality of emitters, and a devicemanufacturing method for evaluating a wafer using such an electron beamapparatus to improve the yield.

Specifically, it is a first object of the present invention to providean electron beam apparatus which is capable of limiting aberration of asecondary optical system without the need for providing a diaphragm inthe optical system.

It is a second object of the present invention to provide an electronbeam apparatus for forming multiple beams using an electron gun whichcomprises a multi-emitter type thermal cathode with small shot noise.

It is a third object of the present invention to provide an electronbeam apparatus which is capable of maintaining consistent emissions ofelectrons and accurately conducting a test using an electron gun whichcan ensure the parallelism between a multi-emitter having a plurality ofemitters and a Wehnelt electrode to readily and accurately align one tothe other.

It is a fourth object of the present invention to provide a devicemanufacturing method which uses such an electron beam apparatus to offera high manufacturing yield.

To achieve the above objects, the present invention provides an electronbeam apparatus for irradiating a first aperture plate having a pluralityof apertures with electron beams emitted from an electron beam source togenerate a plurality of primary electron beams, directing the primaryelectron beams onto a sample, separating secondary electrons emittedfrom the sample from a primary optical system to form a plurality ofsecondary electron beams, and directing a plurality of secondaryelectron beams into a secondary optical system as groups of secondaryelectrons for guiding to a detector, and outputting a detection signalof the secondary electron beams from the detector, wherein the electronbeam apparatus comprises a plurality of apertures corresponding to theplurality of secondary electron beams in front of an incident plane ofthe detector.

In one embodiment of the present invention, the plurality of primaryelectron beams and the plurality of secondary electron beams arearranged in the vicinity of an optical axis, and the plurality ofapertures are formed in the shape of an ellipse which is longer in aradial direction, an X-axis direction of XY-coordinates, and/or a Y-axisdirection of the XY-coordinates from the optical axis in a planeorthogonal to the optical axis.

The electron beam apparatus may further comprise a number of memoriestwice as much as the number of the detectors for storing digital signalsgenerated by A/D converting the detection signals, and change-overswitches disposed in front of and at the back of the memories, whereinthe detection signals from the detectors in one scanning operation areinput in one of the memories while the previous detection signals storedin another of the memories can be transmitted into a CPU or an imageprocessing unit.

In one embodiment of the present invention, it is necessary to deflectthe plurality of secondary electron beams such that they do not move onthe second aperture plate in synchronism with scanning of the pluralityof primary electron beams.

Also, to achieve the above objects, the present invention provides anelectron beam apparatus for narrowly converging a plurality of electronbeams, simultaneously scanning the electron beams on a sample,accelerating secondary electrons from each scanned region of the sampleusing an objective lens, narrowly converging the secondary electrons,separating the secondary electrons from a primary optical system by anExB separator, increasing the interval between the narrowly convergedsecondary electrons using at least one stage of lens after theseparation, and guiding the secondary electrons to a number of secondaryelectron detectors corresponding to the number of beams, wherein anaperture plate having a plurality of apertures, the diameter of which isdetermined to prevent different groups of secondary electrons fromintroducing thereinto, is disposed in front of the secondary electrondetectors.

Also, to achieve the above objects, the present invention provides anelectron beam apparatus for evaluating a sample. The electron beamapparatus comprises means for emitting electron beams, a first apertureplate for forming a plurality of primary electron beams arranged in acircumferential direction about an optical axis of a primary opticalsystem, scanning means for simultaneously scanning the plurality ofprimary electron beams on the sample, means for forming a plurality ofsecondary electron beams separated from the primary optical system andarranged in a circumferential direction about an optical axis of asecondary optical system from secondary electrons emitted from thesample, a detector for detecting the plurality of secondary electronbeams, and a second aperture plate having a plurality of apertures,disposed on one side of the detector on which the secondary electronbeams are incident, wherein the apertures of the second aperture plateare formed to permit secondary electron beams associated therewith topass therethrough and to prevent secondary electron beams not associatedtherewith from passing therethrough.

In one embodiment of the present invention, the second apertures have,for example, a circumferential dimension reduced about an optical axisof the secondary optical system, thereby preventing adjacent secondaryelectron beams not associated therewith from passing therethrough. Theapertures of the second aperture plate may be reduced in a dimension ina non-scanning direction orthogonal to a primary electron beam scanningdirection.

Also, to achieve the above objects, the present invention provides anelectron beam apparatus which is characterized by comprising a thermalcathode which is machined by electric discharge machining using adischarge machining electrode formed with a plurality of recesses of apredetermined size at predetermined positions.

In one embodiment of the present invention, a surface under machining ofthe thermal cathode subjected to the electric discharge machining ispolished to a mirror-smooth state prior to the electric dischargemachining.

Also, to achieve the above objects, the present invention provides anelectron beam apparatus comprising a thermal cathode which is machinedby electric discharge machining using a discharge machining electrodeformed with one or a plurality of recesses of a predetermined size atpredetermined positions, wherein the thermal cathode comprises aplurality of bumps which are created by electric discharge machining atpositions corresponding to the recesses on the surface under machining,and each of the bumps has a mirror polished top surface.

In one embodiment of the present invention, the top surfaces of theplurality of bumps are mechanically polished or chemically mechanicallypolished after the electric discharge machining.

Also, to achieve the above objects, the present invention provides anelectron beam apparatus comprising an electron gun which includes amulti-emitter machined as a cathode including a plurality of integratedemitters, heater for heating the multi-emitter, fixing means for fixingthe multi-emitter and the heater at given positions, a Wehneltelectrode, and a fine adjustment mechanism for finely adjusting theposition of a portion of the Wehnelt electrode which is adjacent to themulti-emitter, wherein the fine adjustment mechanism is configured to beable to finely adjust the portion of the Wehnelt electrode in at leastone of an x-direction, a y-direction, and a θ-direction in a planeparallel to a plane which includes the multi-emitter, and a tiltdirection in a plane perpendicular to the plane.

In one embodiment of the present invention, the portion of the Wehneltelectrode has a plurality of small holes corresponding to the pluralityof emitters, and advantageously has a thickness of 200 μm or less onlyin the vicinity of the holes.

Also, to achieve the above objects, the present invention provides anelectron beam apparatus comprising an electron gun having a cathodemember, a Wehnelt member, and an anode member, wherein a portion of theWehnelt member adjacent to the cathode member can be separated from therest of the Wehnelt member, and can be finely moved in an x-, a y-, or az-direction orthogonal to one another.

In one embodiment of the present invention, the electron beam apparatusforms a plurality of reduced electron beams from emissions of any of theelectron guns, scans a sample surface with the electron beams, anddetects secondary electron beams formed of secondary electrons emittedfrom scanned positions on the sample surface using a plurality ofdetectors.

Further, to achieve the above objects, the present invention provides adevice manufacturing method for evaluating a wafer after the end of eachwafer process for at least one wafer process using any of theaforementioned electron beam apparatus.

The above and other objects and features of the present invention willbecome more apparent from the following detailed description when readwith reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view schematically illustrating anexemplary conventional electron gun;

FIG. 2 is a diagram schematically illustrating the configuration of afirst embodiment of an electron beam apparatus according to the presentinvention;

FIG. 3 is a plan view schematically illustrating exemplary apertures ofa second multi-aperture plate which can be applied to the firstembodiment illustrated in FIG. 2;

FIG. 4 is a plan view schematically illustrating other exemplaryapertures of the second multi-aperture plate which can be applied to thefirst embodiment illustrated in FIG. 2;

FIG. 5 is a plan view schematically illustrating further exemplaryapertures of the second multi-aperture plate which can be applied to thefirst embodiment illustrated in FIG. 2;

FIG. 6A is a diagram schematically illustrating the configuration of asecond embodiment of the electron beam apparatus according to thepresent invention;

FIG. 6B is a plan view showing a positional relationship betweenapertures formed through two multi-aperture plates in FIG. 6A;

FIGS. 7A to 7C are diagrams illustrating the structure of amulti-emitter type thermal cathode for use in the electron beamapparatus illustrated in FIG. 6A, wherein FIG. 7A is a plan view, FIG.7B is a side view, and FIG. 7C is an enlarged view of a bump;

FIGS. 8A and 8B are diagrams illustrating the structure of dischargemachining electrode for manufacturing the thermal cathode illustrated inFIGS. 7A to 7C, wherein FIG. 8A is a plan view, and FIG. 8B is across-sectional view;

FIG. 9A is a plan view of a material under machining which is formedwith bumps in XY-directions, and FIG. 9B is a cross-sectional view takenalong a line B-B in FIG. 9A;

FIG. 10 is a diagram schematically illustrating an exemplary electrongun which can be used in the electron beam apparatus according to thepresent invention;

FIG. 11 is a flow diagram illustrating an exemplary semiconductor devicemanufacturing method using the electron beam apparatus according to thepresent invention; and

FIG. 12 is a flow diagram illustrating a lithography process which isthe core of a wafer processing process in FIG. 11.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following, several embodiments of an electron beam apparatusaccording to the present invention will be described with reference tothe drawings. FIG. 2 is a diagram schematically illustrating a firstembodiment of the electron beam apparatus according to the presentinvention. In FIG. 2, the electron beam apparatus 200 comprises anelectron gun 201 as an electron beam source. An electron beam emittedfrom an emitter 202 of the electron gun 201 is converged by a condenserlens 203 to form a cross-over at a point 205.

A first multi-aperture plate 204 having, for example, four apertures isdisposed below the condenser lens 203. The four apertures of the firstmulti-aperture plate 204 are formed along the circumference of the firstmulti-aperture plate 204. Therefore, the electron beam emitted from theelectron gun 201 is irradiated to the first multi-aperture plate 204 toform four primary electron beams PB about an optical axis W.

In this way, in this embodiment, the electron gun 201 and firstmulti-aperture plate 204 make up an electron beam forming apertures forforming four primary electron beams PB about the optical axis W. Dottedlines represent how an aperture image is focused.

Each of the four primary electron beams PB formed by the firstmulti-aperture plate 204 is reduced by a reducing lens 206, and focusedon a sample 209 by an objective lens 208. The four primary electronbeams PB exiting from the first multi-aperture plate 204 are deflectedto simultaneously scan on the surface of the sample 209 by anelectrostatic deflector 207 disposed between the reducing lens 206 andobjective lens 208 and by an electromagnetic deflector 211 of an ExBseparator 210.

A plurality of focused primary electron beams PB are irradiated onto thesample 209 to scan four points thereon. Secondary electrons emitted fromthese irradiated four points are accelerated by an electric fieldapplied between the objective lens 208 and sample 209. In this way, thesecondary electrons, which are emitted at a large angle to the normal ofthe sample surface, are also finely converged into four fine secondaryelectron beams SB (i.e., four fine secondary electron beams) which passthrough the objective lens 208 and are deflected by the ExB separator210. Thus, the secondary electron beams SB are separated from theprimary optical system for irradiating the sample 209, and introducedinto a secondary optical system.

The secondary optical system has magnifying lenses 212, 213. The foursecondary electron beams SB, which have passed through these magnifyinglenses 212, 213, are spaced from adjacent beams at wider intervals by alarger spacing, focused on four apertures of a second multi-apertureplate 214, and guided to four detectors 215. The second multi-apertureplate 214 is disposed in front of an incident plane of the detectors215. The four apertures of the second multi-aperture plate 214 areformed along the circumference of the second multi-aperture plate 214,and correspond one-to-one to the four apertures formed through the firstmulti-aperture plate 204. Consequently, the four primary electron beamsPB and four secondary electron beams SB are distributed over thecircumference about the optical axis W.

A deflector 216 is also disposed between the magnifying lens 213 andsecond multi-aperture plate 214. The four secondary electron beams SB,which have passed through the magnifying lens 213, can be deflected bythe deflector 216 in synchronism with the scanning of the four primaryelectron beams PB such that they do not move on the secondmulti-aperture plate 214.

Each of the detectors 215 outputs the focused secondary electron beam toan amplification/processing unit 217 in the form of an electric signalindicative of its intensity (detection signal of the secondary electronbeam) The amplification/processing unit 217 amplifies received electricsignals by amplifiers (not shown), respectively, and then converts themfrom analog signals to digital signals by A/D converters (not shown) forstorage in memories 218. The number of memories 218 for secondaryelectronic signals is twice the number of multiple beams. The digitalsignals output from the A/D converters of the amplification/processingunit 217 are handled such that while a signal corresponding to onescanning operation is input to one side (designated by “0”) of thememories 218, a signal stored on the other side (designated by “1”) ofthe memories 218 is read into a CPU 219 or processed thereby, and upontermination of the scanning, change-over switches 220, 221, disposed infront of and at the back of the memories 218 are switched to input asignal produced in the next scanning operation to the memory “1” andsend a signal in the memory “0” to the CPU 219.

The CPU 219 is supplied with a scanning signal for deflecting theprimary electron beam PB applied to the electromagnetic deflector 211.The CPU 219 synthesizes image data from this scanning signal and theaforementioned digital signals, so that an image forming unit, notshown, can construct or display an image representative of a scannedsurface of the sample 209.

Defects on the sample 209 can be detected by comparing the image datawith reference image data of the sample which is free from defects.

Also, a pattern under measurement of the sample 209 is moved close tothe optical axis W of the primary optical system through registration(alignment calibration), and scanned in lines to extract a line widthevaluation signal which can be calibrated as appropriate to measure aline width of the pattern on the sample 209.

Next, description will be made on the apertures of the secondmulti-aperture plate 214 which is a feature of this embodiment. Asdescribed above, since the second optical system is not provided with adiaphragm for limiting aberration of the optical system, beam spots ofthe four secondary electron beams SB have large extent on the secondmulti-aperture plate 214. Therefore, a certain aperture of the secondmulti-aperture plate 214 is passed by another secondary electron beam inaddition to a secondary electron beam which should pass through theaperture, possibly causing so-called cross-talk. To solve this problem,the present invention forms the four apertures of the secondmulti-aperture plate 214 such that different groups of secondaryelectron beams (in other words, secondary electron beams notcorresponded or not associated therewith) will not introduce thereinto.

In FIG. 3, reference numeral 301 designates a position at which aprincipal ray of secondary electron beams is incident on the secondmulti-aperture plate 214; reference numeral 302 designates an apertureof the second multi-aperture plate 214; and reference numeral 303designates a beam receiving surface of the detector 215.

As illustrated in FIG. 3, the four apertures 302 of the secondmulti-aperture plate 214 can be formed in the shape of an ellipse whichis longer in a radial direction from the optical axis in a planeperpendicular to the optical axis. Stated another way, thecircumferential dimension is reduced about the optical axis of thesecondary optical system. In this way, adjacent secondary electronbeams, not associated with an aperture, can be prevented from passingthrough the aperture. Simultaneously, it is possible to efficientlycollect secondary electrons which spread in the radial direction due toaberration. Outside the secondary electron beam incident position 301 inthe radial direction, different groups of secondary electron beams willhardly introduce into the aperture. Thus, even if the apertures 302 areformed in the shape of an ellipse which is longer in the radialdirection from the optical axis as described above, it is sufficientlyunlikely that different groups of secondary electron beams introduceinto apertures 302 not associated therewith.

When the second multi-aperture plate 214 is formed with the fourapertures 302 as illustrated in FIG. 3, the deflection must be providedby the deflector 216 disposed between the magnifying lens 213 and secondmulti-aperture plate 214, such that four secondary electron beamspassing through the magnifying lens 213 will not move on the secondmulti-aperture plate 214 in synchronism with the scanning of the fourprimary electron beams, as described above. In this way, even if theprimary electron beams are scanned, the respective secondary electronbeams can exactly pass through only the apertures 302 associatedtherewith.

Also, as illustrated in FIG. 4, the four apertures of the secondmulti-aperture plate 214 must be formed along the circumferentialdirection of the second multi-aperture plate 214, such that they arearranged at equal intervals when projected onto the X-axis. The fourapertures 402 of the second multi-aperture plate 214 illustrated in FIG.4 are formed in the shape of an ellipse which is longer in the X-axisdirection on the XY-coordinates in a plane perpendicular to the opticalaxis. For example, when a sample stand is continuously moved in theY-direction while four primary electron beams are scanned in theX-direction, different groups of secondary electron beams can beprevented from introducing into the apertures 402 not associatedtherewith, even if the primary beams are scanned in the X-axisdirection, by forming the apertures 402 of the second multi-apertureplate 214 in the shape of an ellipse which is longer in the X-axisdirection. In other words, since the apertures 402 of the secondaperture plate 214 are reduced in the dimension in the non-scanningdirection orthogonal to the scanning direction of the primary electronbeams, it is unlikely that different groups of secondary electron beamswill introduce into apertures 402 not associated therewith.

In addition, the apertures 402 are formed in the shape of an ellipsewhich is longer in the X-axis direction, so that even if four primaryelectron beams are scanned in the X-direction, secondary electron beamswill move within the respective apertures 402 along the X-directionwithout exceeding the apertures 402, with the result that the deflector206 is not necessarily provided. In FIG. 4, 403 indicates a range inwhich the secondary electrons move in synchronism with scanning.

The apertures of the second multi-aperture plate 214 may beadvantageously formed in the shape of an ellipse which is longer in theY-axis direction on the XY-coordinates in the plane orthogonal to theoptical axis. Also, while the second multi-aperture plates 214illustrated in FIGS. 3 and 4 are formed with four apertures 302, 402,respectively, the number of the apertures 302, 402 are not limited tofour, but may be more than that.

FIG. 5 illustrates the second multi-aperture plate 214 which is formedwith eight apertures 502. In this example, as illustrated in FIG. 5, theapertures 502 are at equal intervals when beams are projected onto thex-axis, so that the distance between the beams is shorter in thex-direction, while the distance between the beams is longer in they-direction. In this event, the apertures 502 are preferably formed inthe shape of an ellipse which has a smaller dimension in the x-directionand a larger dimension in the y-direction. The eight apertures 502 ofthe second multi-aperture plate 214 are formed along the circumferencedirection of the second multi-aperture plate 214 in correspondence toeight secondary electron beams positioned on the circumference about theoptical axis, such that they are at equal intervals when projected ontothe y-direction.

Thus, a plurality of apertures of the second multi-aperture plate 214are formed to prevent different groups of secondary electron beams fromintroducing into apertures not associated therewith, thereby making itpossible to prevent the occurrence of cross-talk and to limit aninfluence of the aberration of the secondary optical system even if theoptical system is not provided with a diaphragm. Further, theelimination of the need for providing the diaphragm can lead to theelimination of the need for providing an aligner for adjustor lenses, analigner for the diaphragm, and an stigmator for the cross-over, therebysimplifying the configuration of the apparatus and reducing the size ofthe overall apparatus.

As will be understood from the foregoing description, in the electronbeam apparatus according to the present invention, a plurality ofprimary electron beams are incident on a sample, secondary electronsemitted from the sample are accelerated and separated from the primaryoptical system to form a plurality of secondary electron beams. Theplurality of secondary electron beams are incident on the secondaryoptical system and focused on detecting means which outputs detectionsignals of the secondary electron beams, wherein a plurality ofapertures are provided in front of the detecting means in correspondenceto the plurality of secondary electron beams. Since the plurality ofapertures are formed to prevent secondary electron beams fromintroducing into the apertures not corresponding thereto, the resultingelectron beam apparatus can prevent the occurrence of cross-talk, limitthe influence of aberration of the optical system, and improve thesecondary electron detection efficiency.

FIG. 6A is a diagram schematically illustrating a second embodiment ofthe electron beam apparatus according to the present invention. In FIG.6A, electron beams emitted from an electron gun 601 of the electron beamapparatus 600 are converged by a condenser lens 602 to form a cross-overat a point 604.

Below the condenser lens 602, a first multi-aperture plate 603 having aplurality of apertures is disposed orthogonal to the optical axis W, forforming a plurality of primary electron beams. Each of the primaryelectron beams formed by the first multi-aperture plate 603 is reducedby a reducing lens 605, focused on and projected onto a point 606, andthen focused on a sample 608 by an objective lens 607. A plurality ofprimary electron beams exiting from the first multi-aperture plate 603are deflected by a deflector 609 disposed between the reducing lens 605and objective lens 607 to simultaneously scan on the surface of thesample 608.

For eliminating the influence of field curvature aberration of thereducing lens 605 and objective lens 607, the first multi-aperture plate603 is formed with small apertures on the circumference which arearranged at equal intervals when they are projected onto the x-axis, asillustrated in FIG. 6B. Circles drawn by dotted lines in FIG. 6Cindicate apertures formed through a second multi-aperture plate 614,later described.

A plurality of points on the sample 608 are irradiated with a pluralityof focused primary electron beams. Secondary electron beams emitted fromthe plurality of irradiated points are accelerated by an electric fieldof the objective lens 607, narrowly converged, deflected by an ExBseparator 610, and introduced into a secondary optical system. Asecondary electron image focuses on a point 611 closer to the objectivelens 607 than the ExB separator 610. This is because each primaryelectron beam has energy of 500 eV on the surface of the sample 608,whereas secondary electron beams merely have energy of several eV. It isdesirable that the secondary electron image is designed around the ExBseparator to reduce deflection chromatic aberration of the ExBseparator.

The secondary optical system has magnifying lenses 612, 613. Thesecondary electron beams, which have passed through these magnifyinglenses 612, 613, focus on a plurality of apertures of the secondmulti-aperture plate 614. Then, through the apertures, the secondaryelectron beams are guided to a plurality of detectors 615. Asillustrated in FIG. 6B, a plurality of apertures formed through thesecond multi-aperture plate 614 disposed in front of the detectors 615correspond one-to-one to a plurality of apertures formed through thefirst multi-aperture plate 603.

Each of the detectors 615 converts a detected secondary electron beam toan electric signal indicative of its intensity. The electric signaloutput from each of the detectors is amplified by an amplifier 616, andthen received by an image processing unit 617 for conversion into imagedata. Since the image processing unit 617 is also supplied with ascanning signal for deflecting the primary electron beams, the imageprocessing unit 617 displays an image which represents the surface ofthe sample 608. Defects on the sample 608 can be detected by comparingthis image with a standard pattern.

In this event, it is necessary to pay special attention to minimize theinfluence of three types of aberration, i.e., distortions, axialchromatic aberration, and field astigmatism produced in the primaryoptical system, when the primary electron beams passing through theapertures of the first multi-aperture plate 603 are focused on thesurface of the sample 608, and secondary electron beams emitted from thesample 608 are guided to the detectors 615. Also, in regard to therelationship between the interval of a plurality of primary electronbeams and the secondary optical system, the cross-talk between aplurality of beams can be eliminated by setting the interval of theprimary electron beams larger than the aberration of the secondaryoptical system.

FIGS. 7A to 7C illustrate the structure of a multi-emitter type thermalcathode 601 of the electron beam apparatus 600 illustrated in FIG. 6A,wherein FIG. 7A is a plan view, FIG. 7B is a side view, and FIG. 7C isan enlarge view of a bump. As illustrated in FIGS. 7A and 7B, aplurality of bumps 701-708 (eight in FIG. 7A) are formed along apredetermined circumference on the top face of the thermal cathode 601by a discharge machining electrode, later described, such that they arearranged at equal intervals, when viewed from a side, as illustrated inFIG. 7B. These bumps 701-708 correspond to the emitters in FIG. 6A,respectively. The thermal cathode 601 is formed with two parallelsurfaces 709 on the back for heating.

As illustrated in an enlarged view of FIG. 7C, a leading end portion ofthe bump 705 comprises a cylinder 705 ₁ and a peak face 705 ₂. Thecylinder 705 ₁ and peak face 705 ₂ will be referred to in thedescription later made in connection with FIGS. 8A and 8B. The remainingbumps 701-704, 706-708 are identical in shape to the bump 705.

FIGS. 8A and 8B are diagrams schematically illustrating the structure ofthe discharge machining electrode 800 for manufacturing the thermalcathode 601 illustrated in FIG. 7A, where FIG. 8A is a plan view, andFIG. 8B is a cross-sectional view taken along a line A-A. In FIGS. 8Aand 8B, the discharge machining electrode 800 comprises a disk 801 madeof a tungsten-silver alloy; and a protrusion 802 provided on the back ofthe disk 801 for attachment to an electric discharge machine (notshown). Conical recesses 803 are formed on the surface of the disk 801at positions corresponding to the bumps 701-708 in FIG. 7A. Each of therecesses 803 is communicated with a small hole 804 for smoothlycirculating an insulating oil during electric discharge machining. Eachof the small holes 804 has a diameter of, for example, 100 microns and avertex angle of, for example, 90 degrees. Reference numeral 805 in FIG.8B designates a hole for coupling a lead wire for applying the dischargemachining electrode 800 with a discharge current.

With the discharge machining electrode 800 illustrated in FIG. 8A, amulti-emitter type thermal cathode 601 can be manufactured in the shapeillustrated in FIG. 7A from an arbitrary conductive material, forexample, a material under machining made of LaB6 single crystal, Ta, Hf.tungsten or the like. Thus, the shape of the bumps 701-708 on thethermal cathode 601 are complimentary to the shape of the recesses 803on the discharge machining electrode 800. However, since the dischargemachining electrode 800 comprises the small holes 804 for circulating aninsulating oil, the peak faces 705 ₂, which have been previouslymirror-machined prior to the electric discharge machining, remain at theleading ends of the bumps 705 to the last, as illustrated in FIG. 7B.The cylinder 705 ₁ in communication with the peak face 705 ₁ is smallerin diameter than the small hole 704 by 10 to 20 microns.

Assuming, for example, that the cylinder 705 ₁ has a height of 20microns, the tolerance for the accuracy in the electric dischargemachining is up to 20 microns in the bump height direction, so that theparallelism may be low between the discharge machining electrode 800 anda material under machining.

As described above, the electric discharge machining is performed withthe discharge machining electrode 800 in an insulating oil, and amaterial under machining, which is to be machined into the thermalcathode, is significantly heated as well, thereby possibly causingcomponents of the insulating oil to diffuse into the surface of thematerial under machining to degrade the performance as the thermalcathode. To avoid the degraded performance, the peak faces (designatedby numeral 705 ₂ in FIG. 7C) of the respective bumps 701-708 may bepolished or chemically mechanically polished by five to ten microns tomake mirror surfaces after the electric discharge machining iscompleted. In this event, it is not necessary to mirror-polish thesurface of the material under machining prior to the electric dischargemachining.

Since the discharge machining electrode 800 as described is used forelectric discharge machining, even a hard and fragile material such asLaB₆ single crystal can undergo the electric discharge machining withoutfail. Of course, the electric discharge machining may be repeated usingan electrode which has only one each of recess 803 and small hole 804.

Taking advantage of the foregoing features of the present invention, alarge number of single cathodes can be readily manufactured. Describingthis with reference to FIGS. 9A and 9B, a material under machiningundergoes the electric discharge machining, using the dischargemachining electrode 800 which has the recesses regularly arranged in theXY-directions to regularly form bumps on the material under machining inthe XY-directions, as illustrated in FIG. 5A. Subsequently, the materialunder machining is cut along vertical and horizontal solid lines in FIG.5A into individual elements, thereby making it possible to produce alarge number of tips only with a single mirror polishing operation.

The electron beam apparatus in the configuration illustrated in FIG. 6Amay be applied as well when a single emitter is used.

As will be understood from the foregoing description, the presentinvention provides particular effects as follows.

(1) A variety of conductive materials can be utilized because theemployed discharge machining electrode can machine even a hard andfragile material for a cathode such as LaB₆.

(2) Since the peaks of the bumps formed by the electric dischargemachining are machined to a mirror-smooth state before or after theelectric discharge machining, the planarity of a material undermachining is not affected by the electric discharge machining.

(3) The respective bumps can be machined such that cylinders are left atthe respective leading ends thereof to provide the bumps with a uniformarea at the leading ends thereof.

(4) The accuracy of the electric discharge machining need not be takeninto consideration.

(5) Even when a single cathode is created, a large number of emitterscan be manufactured through a single mirror polishing operation and asingle electric discharge machining operation.

FIG. 10 is a vertical sectional view schematically illustrating anexemplary electron gun which can be used in the electron beam apparatusillustrated in FIG. 6A. The electron gun 900 comprises a cylindricalelectron gun body 901 which is positioned below an anode member 1000,and an insulating ceramics 902 is fixed inside of the electron gun body901. A bottom face of a ceramic seat 903 is fixed on a top face of theinsulating ceramics 902. A multi-emitter 905 is fixed on the ceramicseat 903 such that the multi-emitter 905 is heated by a heater 904. Themulti-emitter 905 is created as a cathode which has a plurality ofemitters 905 a integrated thereon.

High voltage cables 906 for the heater 904 and cathode extend from thebottom face of the insulating ceramics 902. Thus, the multi-emitter 905and heater 904 are fixed in the electron gun body 901 by the ceramicseat 903 and insulating ceramics 902. Therefore, these members can beassembled into a structure which ensures a sufficient rigidity. Also, byaccurately machining the respective members, the parallelism of themulti-emitter 905 to the electron gun body 901 is also fixed at a strictvalue.

As the multi-emitter 905 has been assembled into the electron gun body901, a Wehnelt electrode 907 is next attached to the electron gun body901. The Wehnelt electrode 907 is comprised of a Wehnelt electrode body907 a, and a multi-aperture plate 907 b which can be separated from theWehnelt electrode body 907 a. The multi-aperture plate 907 b can befitted into an opening at a leading end of the Wehnelt electrode body907 a.

The multi-aperture plate 907 b is circular, and has a plurality of smallholes corresponding to the respective emitters 905 a of themulti-emitter 905, i.e., multi-Wehnelt holes 907 c. The multi-apertureplate 907 b has a thickness of 200 μm or less, and preferably 100 μm orless in the vicinity of the multi-Wehnelt holes 907 c, and has a thickerperiphery because screws of a fine adjustment mechanism, laterdescribed, are abutted thereto.

Initially, the Wehnelt electrode body 907 a alone is attached to theelectron gun body 901. The cylindrical Wehnelt electrode body 907 a isrotatably fitted on the similarly cylindrical electron gun body 901. Asthe Wehnelt electrode body 907 a reaches a desired axial position withrespect to the multi-emitter 905, the Wehnelt electrode body 907 a isfixed by a stop ring 908.

Next, the multi-aperture plate 907 b is attached to the opening at theleading end of the Wehnelt electrode body 907 a. The multi-apertureplate 907 b is located adjacent to the multi-emitter 905 within theWehnelt electrode 907. The multi-aperture plate 907 b is attached to theperiphery of the opening by axial screws 909 provided at threelocations. A leading end of a horizontal screw 911 abuts to theperipheral surface of the multi-aperture plate 907 b. The horizontalscrew 911 extends through a plurality of lugs 910 provided on theWehnelt electrode body 907 a.

These screws 909, 911 make up a fine adjustment mechanism for finelyadjusting the position of the multi-aperture plate 907 b. Specifically,as all of the three screws 909 are rotated by the same amount whileviewing them with a microscope, the position of the multi-aperture plate907 b can be finely adjusted in the z-direction, i.e., in the directionperpendicular to a plane which includes the multi-emitter 905. As thethree screws 909 are individually manipulated, the position of themulti-aperture plate 907 b can be finely adjusted in a tilt direction,i.e., in a tilt direction in a plane perpendicular to the plane whichincludes the multi-emitter 905. Through the fine adjustment in the tiltdirection, the parallelism of the multi-aperture plate 907 b can beadjusted with respect to the multi-emitter 905.

Also, by rotating the Wehnelt electrode body 907 a relative to theelectron gun body 901, the position of the multi-aperture plate 907 bcan be finely adjusted in a θ-direction, i.e., the rotating direction ina plane parallel to the plane which includes the multi-emitter 905. Thisrotatable configuration of the Wehnelt electrode body 907 a also formspart of the fine adjustment mechanism.

Further, as the screws 909 are individually operated, the position ofthe multi-aperture plate 907 b can be finely adjusted in the x-directionand y-direction in a plane parallel to the plane which includes themulti-emitter 905 to align the multi-Wehnelt holes 907 c to a pluralityof emitters 905 a of the multi-emitter 905.

In this way, the electron gun body 901, multi-emitter 905, andmulti-aperture plate 907 b can be assembled with a high accuracy whilemaintaining a high parallelism to one another, and the multi-Wehneltholes 907 c can be readily aligned to the associated emitters 905 a. Asa result, the respective emitters 905 a can operate substantially in thesame manner to limit variations in emission current within apredetermined range. Since the multi-aperture plate 907 b is attachedafter the Wehnelt electrode body 907 a is assembled, it is quiteunlikely to destruct the multi-emitter 41 during the assembly.

Further, since the mechanism for adjusting the tilt of the Wehnelt planeto the emitter plane is located at the same z-position as the emitterplane or Wehnelt plane, they will not go out of alignment in the x-, y-or z-direction when the tilt is adjusted.

As will be understood from the foregoing description, according to theelectron gun of the present invention, the multi-emitter and Wehneltelectrode can be accurately assembled and readily aligned to each otherwhile the parallelism is maintained to each other. As a result, therespective emitters can operate substantially in the same manner tolimit variations in emission current within a predetermined range. Also,the Wehnelt electrode can be separated into the body and multi-apertureplate, so that the aperture plate adjacent to the multi-emitter can beattached after the assembly of the Wehnelt electrode body, therebysignificantly reducing the possibility of destructing the multi-emitterthrough contact. Further, as the multi-emitter, heater and insulatingceramics are previously fixed with respect to the electron gun body, anassembling time can be reduced, and a discrepancy can be substantiallyeliminated during an adjustment and after heating. In addition, when theelectron gun of the present invention is used, the electron beamapparatus can be safely operated with multiple beams.

The electron beam apparatus according to the present invention describedabove can be used to evaluate samples in a semiconductor devicemanufacturing method. FIG. 11 is a flow chart illustrating an example ofsuch a semiconductor device manufacturing method. The manufacturingprocesses include the following main processes:

(1) a wafer manufacturing process for manufacturing wafers (or a waferpreparing process for preparing wafers) (process 1001);

(2) a mask manufacturing process for manufacturing masks for use inexposure (or a mask preparing process for preparing masks) (process1002);

(3) a wafer processing process for performing required processing on thewafer (process 1003);

(4) a chip assembling process for dicing the wafer into individual chipsand making each chip operable (process 1004); and

(5) a chip testing process for testing the finished chips (process1005).

Each of these processes includes several sub-processes. Among theseprocesses, the wafer processing process 1002 exerts a deterministicinfluence on the performance of semiconductor devices. In this process,designed circuit patterns-are laminated in sequence on a wafer to form amultiplicity of chips which may operate as a memory or a microprocessorunit. The wafer processing process includes the following processes:

(1) a thin film forming process for forming dielectric thin films whichserve as an insulating layer, a metal thin film for forming wires orelectrodes, and the like (using CVD, sputtering and the like);

(2) an oxidizing process for oxidizing the thin film layers and a wafersubstrate;

(3) a lithography process for forming a resist pattern using masks(reticles) for selectively processing the thin film layers, wafersubstrate and the like;

(4) an etching process for processing the thin film layers and wafersubstrate in accordance with the resist pattern (using, for example, adry etching technique);

(5) an ion/impurity injection and diffusion process;

(6) a resist removing process; and

(7) a testing process for testing the processed wafer.

The wafer processing process is repeatedly executed as many times as thenumber of required layers in order to manufacture semiconductor deviceswhich operate as designed.

FIG. 12 is a flow chart illustrating the lithography process which isthe core of the wafer processing process 1002 in FIG. 11. Thelithography process includes:

(1) a resist coating step for coating a resist on the wafer formed withcircuit patterns in the previous process (step 1101);

(2) an exposure step for exposing the resist (step 1102);

(3) a developing step for developing the exposed resist to create aresist pattern (step 1103); and

(4) an annealing step for annealing the developed resist pattern forstabilization (step 1104).

Since the semiconductor device manufacturing process, wafer processingprocess, and lithography process are well known in the art, furtherdescription thereon is omitted here.

The electron beam apparatus according to the present invention, whenapplied to the aforementioned testing process (7), permits a test to beconducted with high throughput even on those semiconductor devices whichhave miniature patterns, thereby providing for a total inspection,increasing the yield rate for products, and preventing shipment ofdefective products.

INDUSTRIAL AVAILABILITY

Since the present invention provides an electron beam apparatus whichcan limit aberration in a secondary optical system, reduce shot noise,and readily and accurately align a multi-emitter to a Wehnelt, held inparallel with each other, the electron beam apparatus is suitable forevaluating samples in a semiconductor manufacturing process.

1. A method of machining a cathode used in an electron gun, comprisingthe steps of: placing an object to be machined, which is a material ofthe cathode, and a machining electrode in an electrically insulatingoil; applying a discharge current to the machining electrode; andmachining the object to be machined to have a shape corresponding to theshape of the machining electrode by means of electric dischargemachining of the machining electrode, wherein the machining electrodecomprises a disk, recesses formed on a front surface of the disk, aprotrusion provided on a back surface of the disk, and a hole formounting a lead wire for applying discharge current to the machiningelectrode, and wherein bumps corresponding to the shape of the recessesof the machining electrode are formed on the object to be machined bymeans of electric discharge machining.
 2. A method according to claim 1,wherein the recesses have small holes formed therein and wherein theinsulating oil is circulated through the small holes.
 3. A methodaccording to claim 1, wherein the object to be machined comprises anyone of LaB6, a single crystal, Ta, Hf and tungsten.
 4. A method to claim3, wherein the disk is comprised of tungsten-silver alloy.
 5. A methodaccording to claim 4, further comprising the step of polishing theobject to be machined to mirror surfaces after the electric dischargemachining is completed.
 6. A method of machining a cathode used in anelectron gun, comprising the steps of: placing an object to be machined,which is a material of the cathode, and a machining electrode in anelectrically insulating oil; applying a discharge current to themachining electrode; and machining the object to be machined to have ashape corresponding to the shape of the machining electrode by means ofelectric discharge machining of the machining electrode, wherein themachining electrode includes a disk, a plurality of recesses provided ona front surface of the disk and arranged regularly in X and Ydirections, a protrusion provided on a back surface of the disk, and ahole for mounting a lead wire for applying discharge current to themachining electrode, and wherein a plurality of bumps corresponding tothe shape of the recesses of the machining electrode are formed on theobject to be machined by means of electric discharge machining.
 7. Amethod according to claim 6, wherein the object to be machined is cutalong two orthogonal directions after the electric discharge machiningto be separated into a plurality of cathodes.
 8. A method according toclaim 7, wherein the object to be machined comprises any one of LaB6, asingle crystal, Ta, Hf and tungsten.
 9. A method according to claim 8,wherein the disk is comprised of tungsten-silver alloy.
 10. A methodaccording to claim 9, further comprising the step of polishing theobject to be machined to mirror surfaces prior to the cutting but afterthe electric discharge machining is completed.
 11. A method according toclaim 10, wherein a small hole is formed in each recess of the machiningelectrode so that the insulating oil is circulated through the smallholes during the electric discharge machining.